


Of Sacrums Sexy

by Rehlia



Category: Undertale (Video Game)
Genre: Bone Lacing, Dominant Reader, Ecto-Penis (Undertale), Established Relationship, F/M, Overstimulation, PWP, Plot What Plot/Porn Without Plot, Praise Kink, Reader Is Not Frisk, Reader-Insert, Sacrum Lacing, Sensitive bones, gender neutral reader, handjob, pillow humping, submissive sans, you wreck the smol skeleton
Language: English
Status: Completed
Published: 2016-09-11
Updated: 2016-09-11
Packaged: 2018-08-14 10:25:35
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,108
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8010043
Author URL: https://archiveofourown.org/users/Rehlia/pseuds/Rehlia
Summary: <blockquote class="userstuff">
              <p>You lace up Sans' sacrum and have way too much fun with what that does to him.</p>
            </blockquote>





	Of Sacrums Sexy

Sans is laying on the bed in front of you already undressed, his kneecaps digging into the softness of the mattress, his pelvis resting on a fluffy pillow, raising it high enough to make it easily accessible for you. 

You stroke your fingers over his coccyx, lightly, feeling the smooth, mildly porous surface of the bone underneath your fingertips. 

“Comfortable?” You ask.

Sans is wriggling slightly, already panting under your touch. 

“y-yeah. go on.”

“Good,” you purr. You pick up the ribbon from where you stashed it next to the bed, running the silky fabric through your fingers, the smooth surface titillating against your skin. You lean forward until your mouth is right next to the entrance of his ear canal. “I’m gonna be real gentle for now, don’t worry.”

His breath quickens, getting deeper and more ragged at your words. 

You work your fingers up and down over his bones, listing off what you’ve read specifically for this session. Median sacral crest. Sacral articular crest. Posterior sacral foramina. Lateral sacral crest. You kiss the side of his skull, listening to the faint moan he can’t hold back as you stroke all the secret, sensitive little spots on his sacrum. 

You move the ribbon against it, dangling the tip of it against the sensitive bone, stroking gently back and forth in the most teasing manner you can manage. 

You have the very tip of the ribbon folded so it will be easier to slide through him.

“Ready?” You whisper to his skull. He’s moving his pelvis back and forth, grinding both against the pillow he’s laying on and against your hand. 

“f-fucking start already…”

You decide you’ve had enough of teasing him and lean back so you can see what you’re doing. The folded tip of the ribbon pushes against the lowest of his posterior sacral foramina, those little holes on his sacrum that are the focus of this special session that you’ve got planned with him. 

The ribbon pushes through the left, lowermost hole on his sacrum and he can’t help himself and lets out a tiny little moan, barely above a hitched breath, the first inkling of pleasure running through his body and setting his bones on fire. 

“hhh - yeah…”

You reach around his sacrum and feel up his anterior sacral foramina, finding the ribbon poking out of one of the lowermost holes and grabbing it with the tips of your thumb and your forefinger. You pull the fabric through the hole and revel in the ragged moan that escapes him as the silky ribbon is drags against the inside of his bones, a secret, hidden spot unreachable by normal means. Only this can touch him there, can grant him the pleasure of having those tiny little spots set ablaze with desire. 

You bring the ribbon against the anterior sacral foramina opposite of where you started and pull it through, going slow so he can feel every little bit of sensation as you lace the fabric through his sensitive bones. 

You’re rewarded for your efforts with a louder moan, raw and vulnerable and badly hidden against the sheets of the mattress he’s burying his head into. 

“You like that?” You breathe against his skull.

He doesn’t have words for you, only more shameful little moans, but you can tell that, yeah, he totally does, he’s enjoying himself _so fucking much_ , it’s amazing to see him come undone like this. 

Sans. 

Calm and collected Sans, always in control of his emotions and expressions, always in control of every little twitch of his body, always acting cool with a pun on his mouth, is becoming unraveled under your touch, his face more open than you’ve ever seen it as he’s overcome by lust. The ravenous expression on his face as he looks back to you sends a spike of heat into your gut, something feral and carnal coiling deep inside you. 

You love seeing him come apart like this. 

“Two holes done,” you whisper to him, filling his ear canal with your voice, your arousal clear by your tone. His breathing quickens, deepens, becomes more and more ragged and out of control. 

You cross the ribbons over his median sacral crest and revel in the wonderful sight, the dainty blue ribbon contrasting against his smooth bone, the lacing of his sacrum so delicate and appetising to behold. 

And you’re not even finished yet. 

You push the ribbon through the second pair of posterior sacral foramina, one after the other, giggling at the twitch he can’t hold back at the sensation. 

“You can rut against the pillow if you need to, you know,” you tell him, your voice still low and sensual against his ear canal. The little pants he’s breathing out at hearing you speak like this are music to your ears. He doesn’t have muscles but you know that if he did, they’d be quivering right now, straining with sheer lust as you do this to him, as you weave the smooth fabric through one of the most sensitive parts of his body. 

“p-please,” he groans, his face contorting, the expression itself practically begging to go on, to continue, to do anything but stopping. You hold the two ends of the ribbon tight in your fingers, pulling on it harder than strictly necessary just to hear that deep, guttural moan again.

“Sure,” you purr into his ear canal. “You want more, you get more. You didn’t really think I was finished yet, did you?” 

You tackle the third pair of holes in his sacrum, pulling the fabric of the ribbon taut as you weave it through the holes in the bone. His entire body shudders as you enjoy the sensation of the silky ribbon and your fingers against the bone of his backside, rocking forwards into the pillow as he simply can’t contain the need for more friction anymore. His small fists are pulling hard on the bedsheet underneath him.

“My poor boy,” you croon. “All pent up. All needy. Come on. Don’t be shy. Let it all out. I’ll make you feel really good.” Your voice is low, smooth and warm, a cadence deliberately chosen to drive him nuts. 

“a-ahh… ffffhhhh…” 

The sounds escaping through his clenched teeth are so, so satisfying. You want more of that, you _need_ more of that, every little breath and moan contributing to your own satisfaction. 

You bring one hand to the front of his pelvis, briefly, and stroke the bone there, reveling in the feeling of magic that is pooling there as he’s no longer able to control himself.

“f-fuck, c’mon, go on, please, fuck -” he whines, slowly losing even more control over his faculties, all the filters over his language long gone. 

You kiss the side of his skull, drag your soft, human lips over rigid, flushed bone, enjoying the smacking sound you produce as you press a multitude of kisses against him. You kiss and lick your way down to his vertebrae, sucking at the point where they connect to his skull and laughing silently at the way he stutters at the sensation, his composure coming further undone by your ministrations. 

“don’t stop, fuck, fuck, please don’t stop, i - ah… ahhh….” 

You fail to suppress your laughter, quiet and full of affection and desire. He sounds so good like this, so erotic, so lewd as all his inhibitions fall away. 

Your eyes linger on his face, drinking in the needy way his features contort at this most gentle of tortures. What if you ramp it up a notch? You drag your fingernails over the rim of one of the little holes of his sacrum and are rewarded with a helpless, raspy moan, a sound that makes you bite your lip as more heat pools into your goin.

He sounds so sexy.

You’ve always liked his voice, those deep, casual, naturally lazy tones, but hearing him moan like this, little puffs of breath and that delicious gasping that you only get to hear when your fingers are working on him, that’s a special treat, something you cherish and enjoy drawing out of him. He’s no longer in control of his voice and _you’re_ the one who caused that loss of control, you’re the only one who can drive him wild like that, who can transform the lazy cadence of his voice into something needy and begging. 

He’s fully humping the pillow by now, his magic having formed a cute little cock, short but thick, especially at the base, the kind of girth that you know drives you crazy when you have it inside you. 

“Three pairs of holes done, two more to go,” you say, your voice light and playful. “You like this, don’t you? You weren’t sure at first, when I suggested this, but look at you now. I wish you could see your face.”

You envelop his thick cock in one of your hands, giving him a stroke and laughing breathlessly at the lewd smacking sound that movement produces. He’s rocking forwards into your hand, desperate to pursue the pleasurable friction you’re granting him, to relieve the heated build up your ministrations on his sacrum have cause him. 

“mmmh… ah…”

You move your hand back and the ribbon pushes against the fourth pair of posterior sacral foramina, the lower parts that you’ve already threaded through him forming a nice, criss-crossing pattern on his bone, like a corset that’s been carefully laced. Such a cute, cute visual. He’s looking so good in this. You should have done this much sooner. 

“Tell me if it gets too much,” you tell him, pushing the ends of the ribbon through two holes at once, making him scream with pleasure. You pause and listen for your established safe word, but it doesn’t come. Instead, you hear him pant and feel him twitch as you pull the ribbon taut once more, the fabric rubbing harshly against the insides of the holes in his bone, one of those secret little places that nobody but you is allowed to play with. 

He’s being so compliant. So pliant under your weaving hands.

“Good boy,” you whisper against his skull. “You’re doing so well. Such a good boy.”

He’s panting at the praise, his mouth hanging open and you can see a trail of saliva trailing from his mouth to the sheets on the mattress. He’s such a slob sometimes, but this is one of the cases where you like that, you like the idea of him losing all control because of what you’re doing to him, of him coming undone under your deft, skillful fingers. 

“One more,” you say, your own voice breathy and hitching by now. It does things to you, seeing him like this. “One more pair and then - “ you shudder and suppress a moan at the mental image. “And then… I’ll tie a little bow - “

He groans and you take a deep breath, your desire overwhelming you and rendering you unable to speak. Sans is in a similar state underneath you, rutting his dripping cock against the pillow again now that you’ve stopped pumping him. So needy. 

You reach forwards and grasp him a little tighter, laughing quietly when you hear him whimper. Just a few quick strokes before you finish your work on his backside.

You bring the ribbon to the last pair of holes in his sacrum. The folded tips tease against the rims of them, making him jitter against the pillow he’s laying on. His fat little cock rocks against the fabric, drawing another guttural, deep moan from him. You push the ribbon forwards and watch as it vanishes into the little holes, both of your hands now solely focused on his sacrum again. His voice grows higher and higher in pitch as you massage the backside of his bone, the ribbon teasing all the places he could never reach by himself. 

One more twist and you’re done. You pull the ribbon through the final pair of anterior sacral foramina until they come out of the posterior sacral foramina and you’re left with two equally long ends. You could tie them into a bow now. 

You don’t.

You can feel Sans squirm beneath you, waiting for you to finish the lacing of his sacrum. 

Instead, you pull on the ribbon that you have so carefully woven through the open spaces in his bones, until the fabric is completely taut across his bones, the fabric digging against the palms of your hands, and Sans is is yelling with the stimulation against this most sensitive of bones.

“n-no… that’s too… ah… fffhh… too much…!!”

You bring your even face closer to his, not letting go of the ribbon as you do so. 

“Too much? Really? You look like you’re enjoying yourself…” You say teasingly, taking in his expression, a tableau of lustful enjoyment and delicate torture. You pay special attention to his words, waiting for a safe word that just doesn’t come.

He moans again, pushing his pelvis up so his median sacral crest rubs against the smooth skin of your hand. You laugh quietly at his desperate motions, feeling nothing but joy as you realize that despite his weak protests, he really, really doesn’t want you to stop. 

“Shh, don’t worry. I’ll give you everything you need. Gonna make you feel real good,” you whisper, taking care to keep your voice on that low, sensual tone you know he enjoys hearing from you. 

You finally take pity on him and move both of your hands to the ends of the ribbon that’s poking out of his uppermost posterior sacral foramina, taking care to keep them pulled tight as you tie them carefully into a neat little bow. 

“There you go,” you say, giving the bow a little flick with your fingers. It looks so cute, the pastel blue of the ribbon contrasting against the stark white of his bone, the crossing pattern you’ve created looking exactly like a corset. Only you know that he feels every little shift of the fabric against his bones, that with every movement he makes, the tickling stimulation is driving him crazy. This isn’t just a garment he wears, this goes right into him, weaves through him in such an intimate way.

You hook your index finger under the ribbon where it crosses and pull, just a little bit, causing the lacing to rub harshly against the holes in his bone. 

“Well?” You ask.

He nearly screams, the sensation entirely to much against such a sensitive part of himself. He’s still not using your established safe word. What a good boy. So persistent. So strong. So desperate for pleasure. 

“th... “ his voice fails him for a second, careening into something high pitched and helpless before he manages to collect himself. “thank… thank you…”

You grab the ribbon tied through his sacrum with your entire hand and pull him back by it until he’s resting in your lap, and this time he does scream with the overstimulation, pleasure and pain mixing at your rough handling of him. 

“Time to have fun,” you tell him. Not that this hasn’t been fun yet already. You keep your hand on the lacing you’ve woven through his sacrum, pulling at it every now and then as you move your other hand to his cock and start pumping him slowly, steadily. His spine arches and his skull falls back, coming to rest on your shoulder, his mouth hanging open as moan after moan escapes him. The wet slapping sound of your hand pumping his cock mixes with his breathy groans into something beautiful. 

After all your teasing, he doesn’t last long.

“f… fhh… fuck…. i’m cum- c-cumming- _ah_ -”

Tears gather at the corners of his eyes as the first drops of his come leave the tip of his cock and then he’s spurting all over your hand, some of it even reaching as high as his rib cage, sullying the pristine whiteness of the bones there. You watch the sticky substance dripping down his ribs in satisfaction, still not easing up, continuing to pump him through his orgasm.

“ _fuck fuck fuck fuck_ -”

“Say the magic word and I’ll stop,” you tell him, pulling at his sacrum lacing again as his pelvis spasms, the heat of his pleasure becoming too much for him to bear. He’s fully crying now, panting and wailing at your ministrations. His toes are curling and the bones of his legs are shaking, cramping, trying to work through the flood of sensations you’re giving him. 

No safe word.

“More?” You ask him.

He doesn’t have enough breath left in him to answer you, his mouth hanging open, drool dribbling down his jaw as he twitches in your hands, the pips of light in his eyes rolling back and shrinking until they’re gone. His small feet dig and dig into the mattress, he’s pushing himself further into you as he tries to get away from your hands, to ease up on the brutal overstimulation you’re putting him through. 

No safe word.

You keep going, tugging at the lacing again and pumping his half-flaccid cock, testing the waters. Maybe you really can get another round in. He’s shouting now, almost screaming, more tears gathering at the rims of his eye sockets but his cock is hard again. You didn’t think he could manage this fast, but then his cock isn’t a normal one, it’s made of magic, and you definitely enjoy his quick recuperation. His leg begins to twitch rhythmically against the wall, like a dog, thumping and thumping faster than even your hands are working, you think he’s trying to say something but he’s so overworked that his words are slurring together. It doesn’t sound like the safe word in any case, so you keep going. His voice reaches higher and higher notes, tones you’ve never heard from him before and then he goes completely rigid. 

“a-aaaaaah…. hh…. _ha - fffhh_ ….”

You hear his spine pop under the strain and then you feel more cum dribble over your hand, a smaller stream of it this time, and suddenly the rigidity in his bones is gone and he goes slack, flopping against you like a ragdoll, like a puppet whose strings have been cut. 

His eye sockets are completely devoid of light and there is no movement on his tortured, blissed out face.

He’s lost consciousness.

“Whoops,” you giggle, gently removing your hands from his cock and his sacrum to start stroking his skull. He deserves it, he’s been so good today. 

You leave the lacing in place though. 

You have a couple more ideas of what to do with that.


End file.
